This invention relates to a speed control device for a transistor motor, and more particularly to a speed control device for a transistor motor capable of being manufactured inexpensively.
For the purpose of controlling the rotation speed of a transistor motor having a stator whose magnetic pole members are wound with field windings and a rotor consisting of a permanent magnet, there has been proposed a speed control device comprising position detectors provided in corresponding relation to the respective magnetic pole members, a plurality of driving transistors respectively connected in series to the field windings and opened and closed by the signals from the position detectors to changeover in turn the current of the field windings, a field current control transistor for controlling in accordance with the speed control signal the amount of current flowing from a DC power source into the respective field windings, and a servo circuit for controlling the field current control transistor in accordance with the outputs from the position detectors. Generally, in the transistor motor, it is demanded that it can be manufactured in low cost and requires as small a control power as possible to facilitate its miniaturization. In the foregoing prior art speed control device, however, the field current control transistor is required to make control of the total amount of field current. Accordingly, the field current control transistor is required to have a capacity for large current and accordingly the servo amplifier of a servo circuit for supplying control power to this field current control transistor is also required to have a capacity for large current. For these reasons, the prior art speed control device for a transistor motor is unsatisfactory in respect of the cost reduction and miniaturization of the transistor motor.
Accordingly, the object of the invention is to provide a speed control device for a transistor motor capable of dispensing with the above mentioned prior art field current control transistor and therefore attaining the cost reduction and miniaturization of the transistor motor.